Insulin is a hormone which allows the liver and other organs to store sugar in the form of glycogen and which also controls the oxidation of sugar within the body of mammals. In diabetic subjects, exercise decreases the requirement for insulin. In normal subjects, acute exercise causes insulinopenia and marked increase of muscle glucose utilization. An increased sensitivity of muscle to insulin during exercise has therefore been postulated. During acute exercise, serum levels of radioimmunoassayable growth hormone(GH) are increased severalfold. Since in vivo administration of GH promotes glucose utilization in rats (insulin-like activity), there is the possibility that the hormone or a fragment of the hormone is involved in the increased sensitivity of muscle to insulin. During a study of the early insulin-like action of human GH, it was observed that the fragment corresponding to amino acid residues 32-46 of the molecule could improve the glucose tolerance in mice (L. G. Frigeri et al., 64th Annual Meeting of the Endocrine Society, Abstract #88).